Self-piercing riveting (SPR) is a spot-joining technique in which a self-piercing rivet is driven, by a punch, into a layered workpiece supported on a die. The die is shaped so that as the rivet is driven into the workpiece towards the die, the material of the workpiece plastically deforms. This flow of workpiece material causes the annular tip of the rivet to flare outwards and remain encapsulated by an upset annulus of the workpiece material. The flared tip of the rivet interlocking with the upset annulus of the workpiece prevents removal of the rivet or separation of the layers of the workpiece.
Insertion of the rivet into the workpiece is performed using a linear actuator, which drives the punch and rivet towards a stationary workpiece and die, or drives the die and workpiece towards a stationary rivet and punch (the former arrangement being more common). Linear actuators of many different types may be used for SPR, but the most common types are hydraulic cylinders, or motor-driven electrical actuators. Motor-driven electrical actuators, unlike electrical actuators such as solenoids, utilise a conventional rotary electric motor. The motor operates a ball screw, lead screw or roller screw mechanism so as to produce linear movement of an actuator output shaft. These three mechanisms all follow the same basic format—the motor rotates a first threaded member which is meshed (directly or indirectly) with a second threaded member connected to the output shaft. If the first and second threaded members rotate in unison, no linear motion is produced. If the first threaded member rotates relative to the second, however, (for instance if the second threaded member is prevented from rotating), rotation of the first threaded member will be translated into linear motion of the second threaded member.
As an example, a lead screw mechanism comprises an externally-threaded screw shaft meshed directly with an internally-threaded nut. If the screw shaft is connected to a motor and the nut to an output shaft, the screw shaft constitutes the first threaded member and the nut the second threaded member. By rotating the screw shaft using the motor, the nut moves along the screw shaft and the output shaft moves linearly. Similarly, if the nut is connected to the motor and the screw shaft to the output shaft, the nut constitutes the first threaded member and the screw shaft the second threaded member. By rotating the nut, the screw shaft moves axially within the nut and the output shaft is extended or retracted linearly. The above also holds in relation to ball screw mechanisms, except that the nut and screw shaft are not meshed directly. Instead, they are meshed indirectly via a set of ball bearings disposed therebetween. Similarly, roller screw mechanisms follow the above principle but the screw shaft and nut are meshed indirectly through a set of threaded roller.
In many applications for linear actuators, such as SPR, it is desirable to limit the stroke length of a linear actuator (i.e. limit the freedom of movement of the actuator's output shaft). In machines where the movement of the actuator output shaft is controlled solely by a control algorithm, there may be potential for a fault in the system to allow overtravel of the output shaft, with potentially severe consequences. For instance, in an SPR tool where the electrical actuator drives the punch, an interruption in the power supply to the tool may prevent the control unit from sending a timely ‘stop’ signal to the actuator. As a result, the actuator may drive the punch beyond its intended final position and into the workpiece itself, spoiling the workpiece.
It is known to use stop surfaces positioned in the path of the output shaft of an actuator so as to prevent overtravel. Although such surfaces can be effective in stopping the movement of an actuator output shaft, impact of the shaft against a stop surface can cause significant damage to the actuator. For instance, impact against stop surfaces can cause the output shaft of an actuator to deform, bringing it out of acceptable dimensional tolerances. The problem of collision with stop surfaces can be particularly severe in applications which utilise not only the force from the actuator, but also the kinetic energy of components moved within the actuator (for example in some SPR tools the rivet insertion force is provided partially by the rotational inertia of a flywheel generating linear movement of the punch). Such applications necessarily utilise relatively heavy components moving at relatively high speeds, and so the damage caused by a collision with a stop surface may be particularly severe.
So as to limit the damage brought about by collision of an actuator output shaft with a stop surface, some actuators utilise elastomeric crash pads positioned over the stop surfaces. During a collision, the crash pads elastically deform and help to dissipate the energy of the collision, reducing the force applied to the components at risk of damage. However, such crash pads can be prone to wear and/or degradation, releasing small particles which can migrate within the tool and cause damage. For example, they can abrade the seals of hydraulic or pneumatic cylinders, or obstruct proper function of threaded components in motor-driven linear actuators. In addition, once a crash pad has been deformed to a certain extent during a collision, it will be incapable of absorbing any further energy. At this point, the actuator output shaft will still experience a ‘hard stop’, and damage may still occur. Replacement of crash pads necessitates opening up the internal workings of an actuator
An additional risk of damage occurring is present if the actuator continues to urge the output shaft to move when the shaft is at the end of its travel. For instance, in an SPR tool which utilises an electric lead screw actuator to drive the punch, rotational inertia in the components of the lead screw mechanism may continue to apply force (axial and/or torsional) to the punch during a collision with a stop surface, after it has reached the ‘hard stop’ of a crash pad. This may bring about excessive loading in the threads of the lead screw due to high torques from sudden deceleration, damaging the threaded components. These are often particularly costly components because they are manufactured to precise tolerances out of very hard material.
A solution to the specific problem of excess loading in the threads of a motor-driven electrical actuator has been proposed in the form of a frangible key assembly mounted to the tip of the actuator output shaft. An actuator with frangible key assembly also has an anti-rotation tube which is fixed to its housing and projects in the direction of movement of the actuator output shaft. The key assembly has a pair of keys projecting from a central hub into corresponding keyways in the anti-rotation tube. The keys are each connected to the hub by a shear pin. In normal use the keys of the key assembly being received within keyways in the anti-rotation tube prevents the key assembly, and thus the output shaft, from rotating. As outlined above, this brings about linear movement of the output shaft. As the output shaft moves, the key assembly moves with it and the keys run along the keyways. If the output shaft is subjected to excessive resistance to motion, however, the force urging the second threaded member to rotate with the first is increased. The key assembly is therefore subjected to increased torsional loading and the shear pins holding the keys in place are fractured. At this point the key assembly (minus the keys) is able to rotate within the anti rotation-tube, and so the output shaft is able to rotate and no further linear movement (and damage therefrom) takes place.
One problem with the above solution is that it requires extremely tight tolerances in relation to the dimensions and hardness of the shear pins. Since in normal use the first threaded member exerts a significant force on the second threaded member, urging it to rotate, the shear pins must be of sufficient size and strength to withstand this loading without fracturing or experiencing fatigue. At the same time, the pins must be small and soft enough that they will reliably fracture before the loading in the threads of the actuators can increase enough for damage to the threaded members to result. By way of an example, in one particular actuator the working torque applied to output shaft, which must be withstood by the shear pins, is 80 Nm. The threaded members of this actuator are able to withstand up to 140 Nm of torque, or 140 kN of axial force, before damage occurs. The pins must therefore reliably fracture when less than 140 Nm of torque is applied to the output shaft, but must withstand 80 Nm of torque without any risk of fatigue. The shear plane of the pins is 2 cm radially outwards, therefore the pins must reliably fracture under a shear load of 7 kN but withstand a shear load of 4 kN without any risk of fatigue. This 3 kN range, within which the pins must transition from completely unaffected to absolute failure, equates to a very tight operating window.